The present invention relates to the papermaking arts including fabrics and belts used in the forming, pressing and drying sections of a paper machine, and to industrial process fabrics and belts, engineered fabrics and belts, along with corrugator belts generally.
The fabrics and belts referred to herein may include those also used in the production of, among other things, wetlaid products such as paper and paper board, and sanitary tissue and towel products made by through-air drying processes; corrugator belts used to manufacture corrugated paper board and engineered fabrics used in the production of wetlaid and drylaid pulp; in processes related to papermaking such as those using sludge filters and chemiwashers; and in the production of nonwovens produced by hydroentangling (wet process), meltblowing, spunbonding, airlaid or needle punching. Such fabrics and belts include, but are not limited to: embossing, conveying, and support fabrics and belts used in processes for producing nonwovens; and filtration fabrics and filtration cloths.
Such belts and fabrics are subject to a wide variety of conditions for which functional characteristics need to be accounted. For example, during the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. The yarns of the fabric that run along the direction of paper machine operation are referred to as the machine direction (MD) yarns; and the yarns that cross the MD yarns are referred to as the cross machine direction (CD) yarns. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long nip presses has been found to be advantageous over the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a cellulosic fibrous web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section. A commonly used type of long nip press is the shoe type long nip press, or “shoe nip press.”
In the shoe nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity with one another, a nip, which can be five to ten times longer in the machine direction than one formed between two press rolls, is formed. This increases the so-called dwell time of the cellulosic fibrous web in the long nip while maintaining an adequate level of pressure per square inch of pressing force. The result of this long nip technology has been a dramatic increase in dewatering of the cellulosic fibrous web in the long nip when compared to conventional press nips on paper machines.
The shoe nip press requires a special belt, such as that taught for example in commonly assigned U.S. Pat. No. 6,465,074 to Fitzpatrick. This belt is designed to protect the press fabric supporting, carrying and dewatering the cellulosic fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
In addition to being useful in shoe nip presses, the present invention also relates to other papermaking, paper-processing, and industrial applications. The present invention contemplates fabrics and belts including forming, press and dryer fabrics, other belts used in papermaking and industrial processes, and other engineered fabrics. In this regard, as part of the manufacturing steps for paper for example, and also for some fabrics, the surface of the paper or a fabric may be smoothed by a calendering process. Calendering can be performed by a belt roll calendar or a shoe nip calendar as well as other methods known to those of skill in the art.
The calendering process smoothes or glazes the paper by pressing it between two rolls or pressing it between a roll and a shoe to smooth, glaze or thin the paper web. In most instances there is also an application of heat to the paper being calendered. An arrangement similar to the long nip press may be employed in calendering the paper web. The paper to be calendered is placed under tension and is compressed or calendered to obtain the desired thickness and gloss characteristics. A belt that is used in such a process is under a number of stresses that require different attributes of the belt to prevent its failure; that is, amongst them being resistance to thermal degradation, resistance to abrasion, and resistance to flexure and compression fatigue. One aspect of the present invention is directed to providing an efficient method of applying materials to a fabric or belt to improve resistance to the failure caused by the environmental factors it will be subjected to during its use.
Industrial fabrics often include a number of layers. The industrial fabric may include a base fabric or support structure as one layer. Often the base fabric may be woven. The fabrics may take the form of an endless loop either by being woven or formed as an endless loop, or by seaming the fabrics into an endless loop.
Fabrics such as press fabrics may have one or more layers of batt fibers applied by needling. Corrugator belts used in corrugator machines to make corrugated paperboard also usually have an endless support structure and one or more layers of batt applied by needling.
Structures to be used as belts in papermaking such as shoe press belts, transfer belts, and calender belts usually will have one or more surfaces coated with a resin to at least partially impregnate the support structure making the belts impervious to water and oil. Other process belts such as some transfer belts may still have a resin coating, but will have either a degree of porosity and/or porosity and permeability to fluids such as water.
In processes associated with the production of paper, these fabrics and belts can wear and in the case of dryer fabrics and calender belts especially, suffer from thermal degradation. For example, in calendering operations the rolls are routinely heated up to 250° C. and in some known applications temperatures of 300° C. are anticipated. These temperatures cause the calender belt surface resin to degrade over time, leading to extensive boundary cracking and potential delamination, limiting its useful life. As a result, fabrics and belts operating under these conditions are in need of thermal protection.
To minimize wear and thermal degradation, papermaking process belts may include an outer synthetic resin layer having improved thermal degradation, abrasion, or resistance to compressive fatigue. For example, current calender belts are composed of a flexible urethane or rubber-like resin layer applied to a reinforcing yarn structure. The elasticity or the hardness of the layer may be adjusted in accordance with the type of resin used. Generally, the lower the hardness, the better the smoothness and gloss of the paper sheet. But when the hardness of the resin is too low, plastic deformation may occur and the life of the belt may be shortened through use. On the other hand, where the hardness of the resin is too great, other problems can be found such as inflexibility, and a shortened belt lifespan due to cracking of the hardened resin.
In general and also by way of background, the production of nonwoven products is also well known in the art. Such products are produced directly from fibers without conventional spinning, weaving or knitting operations. Instead, they may be produced by spunbonding or meltblowing processes in which newly extruded fibers are laid down on an engineered fabric to form a web while still in a hot, tacky condition following extrusion, whereby they adhere to one another to yield an integral nonwoven web. Nonwoven products may also be produced by air-laying or carding operations where the web of fibers is partially consolidated, subsequent to deposition a second operation such as needling or hydroentangleing which produces the final nonwoven product. In the latter, high-pressure water jets are directed vertically down onto the web to entangle the fibers with each other. In needling, the entanglement is achieved mechanically through the use of a reciprocating bed of barbed needles which force fibers on the surface of the web further thereinto during the entry stroke of the needles. The support fabric for all these processes are exposed to some degree of frictional abrasion. Also the belts and fabrics may partially fill with contaminants. These contaminants are typically particles of the particular manufacturing process such as particles of the polymer itself, lattices, additives, etc., that adhere to the surface of the fabric and need to be removed.
Corrugator belts run on a corrugator machine and are used to manufacture corrugated paper board. These belts are exposed to a hot, wet environment as well as abrasion as they pass over stationary elements. Surface contamination, particularly with starch, is also a problem.
Due to the severe operating environment in which many fabrics and belts operate, the above considerations need to be taken into account to achieve desired functional characteristics. In one aspect of the present invention a surface layer is applied to the fabric or belt, which layer can be organic or inorganic, and is applied via thermal spray that will enhance its desired properties.
Accordingly, there is a need for fabrics and belts having improved functional characteristics. Further, there is a need for an improved method of applying materials to fabrics and belts to achieve these goals in an efficient and economical fashion.